libfc: extend ex_lock to protect all of fc_seq_send

This warning was reported recently:

WARNING: at drivers/scsi/libfc/fc_exch.c:478 fc_seq_send+0x14f/0x160 [libfc]()
(Not tainted)
Hardware name: ProLiant DL120 G7
Modules linked in: tcm_fc target_core_iblock target_core_file target_core_pscsi
target_core_mod configfs dm_round_robin dm_multipath 8021q garp stp llc bnx2fc
cnic uio fcoe libfcoe libfc scsi_transport_fc scsi_tgt autofs4 sunrpc
pcc_cpufreq ipv6 hpilo hpwdt e1000e microcode iTCO_wdt iTCO_vendor_support
serio_raw shpchp ixgbe dca mdio sg ext4 mbcache jbd2 sd_mod crc_t10dif pata_acpi
ata_generic ata_piix hpsa dm_mirror dm_region_hash dm_log dm_mod [last unloaded:
scsi_wait_scan]
Pid: 5464, comm: target_completi Not tainted 2.6.32-272.el6.x86_64 #1
Call Trace:
[<ffffffff8106b747>] ? warn_slowpath_common+0x87/0xc0
[<ffffffff8106b79a>] ? warn_slowpath_null+0x1a/0x20
[<ffffffffa025f7df>] ? fc_seq_send+0x14f/0x160 [libfc]
[<ffffffffa035cbce>] ? ft_queue_status+0x16e/0x210 [tcm_fc]
[<ffffffffa030a660>] ? target_complete_ok_work+0x0/0x4b0 [target_core_mod]
[<ffffffffa030a766>] ? target_complete_ok_work+0x106/0x4b0 [target_core_mod]
[<ffffffffa030a660>] ? target_complete_ok_work+0x0/0x4b0 [target_core_mod]
[<ffffffff8108c760>] ? worker_thread+0x170/0x2a0
[<ffffffff810920d0>] ? autoremove_wake_function+0x0/0x40
[<ffffffff8108c5f0>] ? worker_thread+0x0/0x2a0
[<ffffffff81091d66>] ? kthread+0x96/0xa0
[<ffffffff8100c14a>] ? child_rip+0xa/0x20
[<ffffffff81091cd0>] ? kthread+0x0/0xa0
[<ffffffff8100c140>] ? child_rip+0x0/0x20

It occurs because fc_seq_send can have multiple contexts executing within it at
the same time, and fc_seq_send doesn't consistently use the ep->ex_lock that
protects this structure. Because of that, its possible for one context to clear
the INIT bit in the ep->esb_state field while another checks it, leading to the
above stack trace generated by the WARN_ON in the function.

We should probably undertake the effort to convert access to the fc_exch
structures to use rcu, but that a larger work item. To just fix this specific
issue, we can just extend the ex_lock protection through the entire fc_seq_send
path

Signed-off-by: Neil Horman <[e-mail]>
Reported-by: Gris Ge <[e-mail]>
CC: Robert Love <[e-mail]>
Signed-off-by: Robert Love <[e-mail]>

drivers/scsi/libfc/fc_exch.c

···
 	fc_exch_release(ep);	/* drop hold for exch in mp */
 }
 
-/**
- * fc_seq_send() - Send a frame using existing sequence/exchange pair
- * @lport: The local port that the exchange will be sent on
- * @sp:	   The sequence to be sent
- * @fp:	   The frame to be sent on the exchange
- */
-static int fc_seq_send(struct fc_lport *lport, struct fc_seq *sp,
+static int fc_seq_send_locked(struct fc_lport *lport, struct fc_seq *sp,
 		       struct fc_frame *fp)
 {
 	struct fc_exch *ep;
···
 	u8 fh_type = fh->fh_type;
 
 	ep = fc_seq_exch(sp);
-	WARN_ON((ep->esb_stat & ESB_ST_SEQ_INIT) != ESB_ST_SEQ_INIT);
+	WARN_ON(!(ep->esb_stat & ESB_ST_SEQ_INIT));
 
 	f_ctl = ntoh24(fh->fh_f_ctl);
 	fc_exch_setup_hdr(ep, fp, f_ctl);
···
 	error = lport->tt.frame_send(lport, fp);
 
 	if (fh_type == FC_TYPE_BLS)
-		return error;
+		goto out;
 
 	/*
 	 * Update the exchange and sequence flags,
 	 * assuming all frames for the sequence have been sent.
 	 * We can only be called to send once for each sequence.
 	 */
-	spin_lock_bh(&ep->ex_lock);
 	ep->f_ctl = f_ctl & ~FC_FC_FIRST_SEQ;	/* not first seq */
 	if (f_ctl & FC_FC_SEQ_INIT)
 		ep->esb_stat &= ~ESB_ST_SEQ_INIT;
+out:
+	return error;
+}
+
+/**
+ * fc_seq_send() - Send a frame using existing sequence/exchange pair
+ * @lport: The local port that the exchange will be sent on
+ * @sp:	   The sequence to be sent
+ * @fp:	   The frame to be sent on the exchange
+ */
+static int fc_seq_send(struct fc_lport *lport, struct fc_seq *sp,
+		       struct fc_frame *fp)
+{
+	struct fc_exch *ep;
+	int error;
+	ep = fc_seq_exch(sp);
+	spin_lock_bh(&ep->ex_lock);
+	error = fc_seq_send_locked(lport, sp, fp);
 	spin_unlock_bh(&ep->ex_lock);
 	return error;
 }
···
 	if (fp) {
 		fc_fill_fc_hdr(fp, FC_RCTL_BA_ABTS, ep->did, ep->sid,
 			       FC_TYPE_BLS, FC_FC_END_SEQ | FC_FC_SEQ_INIT, 0);
-		error = fc_seq_send(ep->lp, sp, fp);
+		error = fc_seq_send_locked(ep->lp, sp, fp);
 	} else
 		error = -ENOBUFS;
 	return error;
···
 	f_ctl = FC_FC_LAST_SEQ | FC_FC_END_SEQ | FC_FC_SEQ_INIT;
 	f_ctl |= ep->f_ctl;
 	fc_fill_fc_hdr(fp, rctl, ep->did, ep->sid, fh_type, f_ctl, 0);
-	fc_seq_send(ep->lp, sp, fp);
+	fc_seq_send_locked(ep->lp, sp, fp);
 }
 
 /**
···
 		ap->ba_low_seq_cnt = htons(sp->cnt);
 	}
 	sp = fc_seq_start_next_locked(sp);
-	spin_unlock_bh(&ep->ex_lock);
 	fc_seq_send_last(sp, fp, FC_RCTL_BA_ACC, FC_TYPE_BLS);
+	spin_unlock_bh(&ep->ex_lock);
 	fc_frame_free(rx_fp);
 	return;